Some of the greatest problems of laying pipe under water are making good seals between pipes in a string of pipe, testing the seals, and moving the bulkheads under the water. Also controlling the amount of pressure differential between the inside and outside of the pipe as not to exceed the design limits of the pipe. An improvement in the method of laying pipes under water is our U.S. Pat. No. 3,952,524, where the incompressability of water is used to help make better connections between adjacent pipes in a string of pipes. The present invention improves still further on that method by providing means to test just how good the joint seals are, easing the burden of moving the bulkhead under the water, and controlling the maximum pressure differential imposed on the pipe.